A solid oxide fuel cell (SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Generally, a ceramic material of yttria stabilized zirconia (YSZ) is commonly being in SOFC as electrolyte, while a material of Ni-YSZ ceramic is the anode material of choice and a material of LaMnO3 is the cathode material.
It is noted that a conventional SOFC generally uses a cement anode material to construct its support element, whereas the more recent metal-supported SOFC is designed with a permeable metal layer to be its support element. However, no matter the support element is made of a cement anode material or is made of a permeable metal, it can be the thickest layer in each individual cell and is generally about 0.5˜1.5 mm in thickness and 5×5˜20×20 cm2 in size. Although a thick layer of support element can provide good mechanical support, the permeability of such support element can be poor that not only it may be difficult for hydrogen to enter the anode layer, but also a water vapor byproduct generated from the electrochemical reaction at the anode layer may not be drained out of anode layer easily. Moreover, if the water vapor byproduct is not being drained out of the anode layer in time, the path allowing hydrogen to be guided into the anode layer can be blocked, resulting a great polarization voltage drop on the anode side, such as a polarization loss in concentration gradient, which is going to further affect the performance of the SOFC.
Current SOFCs usually are built with comparatively thinner anode layer, cathode layer and electrolyte layer for reducing the polarization losses and ohmic losses in electrodes and electrolyte respectively. Thus, it is generally required to have a support element to be designed in the fuel cell structure. However, a SOFC with thick support element may have good mechanical support, but can be poor in permeability that results in low cell output.
Although the permeability of the support element in SOFCs can be improved simply by reducing the thickness of the support element, it is noted that the strength of the support element may be weakened correspondingly, and consequently the long-term operation stability of the cell structure can be adversely affected.
Conventionally, a permeable metal substrate is formed of stacking granular powders by powder metallurgy, and the binding between granular powders is achieved by a pressing process and a high temperature sintering process so as to form a permeable metal substrate with sufficient mechanical strength while allowing pores to exist between granular powders for providing permeability. However, since the stacking of the granular powders is disorderly, irregular and uncontrollable, the gas channels formed by the connection between pores between granular powders can be irregular and tortuous. Consequently, such tortuous gas channels are not good for gas flows, including hydrogen and water vapor flows.
In addition, as the sizes of the pores that are being formed between granular powders are also uncontrollable, the sizes of the gas channels that are the direct result of serially connected pores are also uncontrollable. Consequently, such tortuous gas channels that are formed with connecting the varying pore sizes may be the cause of resistances to the hydrogen and water vapor flows in the fuel cell, resulting that not only it may be difficult for hydrogen to enter the anode layer, but also the water vapor byproduct generated from the electrochemical reaction at the anode layer may not be drained out of anode layer easily.
According to the foregoing description, it is noted that the tortuous gas channels that are formed of varying pore sizes may also cause the corresponding permeable metal substrate to have low permeability and the substrates manufactured by the same process have varying magnitudes of permeability. Therefore, the output powers of fuel cells that use the aforesaid permeable metal substrates as their support elements may be various too.
Moreover, since the mechanical strength of permeable metal substrate is determined by the binding strength between the stacking granular powders, and the necking portions between granular powders can most often be exposed in the environment of manufacturing process or to the cell working environment where the metallic properties of substrates can be changed due to the chemical transformation or oxidization of the necking portions, this results in causing the whole metal substrate to become brittle, unable to bear impact or thermal shock, and therefore unable to produce functional layers in the SOFCs, so that the materials of stacking granular powders should be carefully selected to have enough resistances to this chemical transformation or oxidization.